1. Field of the Invention
The present invention relates generally to a highly stable current source, which may be utilized in any suitable application requiring a stable current source, such as, for example, in an ohms converter circuit for measuring the resistance of an unknown resistor. More particularly, the subject invention pertains to a highly stable current source in which the actual output current of the current source is monitored to ensure its accuracy. The circuit of the present invention is not temperature dependent, and is protected against the accidental application thereto of high voltages.
2. Discussion of the Prior Art
In a number of electrical control and supply circuits and systems in the prior art, it is highly desirable, or even necessary for satisfactory performance, that the current supplied to an operating element or load be substantially constant, unaffected by temperature changes or by voltage or current variations in the circuit or system, and essentially independent of the impedance of the element or load. One such prior art circuit concerns an ohm's converter circuit for measuring the resistance value of an unknown resistor.
In a digital multimeter, an ohms measurement is commonly performed by measuring the voltage drop produced across an unknown resistor with the application thereto of a known current flow. In order to get an accurate reading, two measurements must be performed simultaneously, namely the voltage drop across and the current flow through the unknown resistor. This measurement technique generally utilizes either a sample and hold circuit or two A/D converters. However, both approaches are generally unacceptable because mismatches and drift create errors in the measurements.
In lieu of performing simultaneous measurements, the approach of an ohms converter circuit can be employed, which utilizes a highly stable current source such as that provided by the present invention. Most ohms converter circuits regulate the voltage drop across the emitter resistor of a transistor, the collector of which is the output. With this approach, the beta of the output transistor changes with temperature and with changes of V.sub.CE, which causes I.sub.C to change, thereby creating an inherent error. The stable current source of the present invention eliminates this basic design flaw by monitoring the actual output current of the current source.
Although the highly stable current source of the subject invention is specifically disclosed in the context of an ohms converter circuit, the present invention is useful in providing a stable current source useful in any general application requiring a highly stable current source.